Typical autofocus systems in a digital camera for capturing still images or video, uses a “through-the-lens” autofocus system that captures a series of 5-20 or more autofocus images taken with a moveable focus lens in different focusing positions. For an autofocus system that includes a liquid crystal focus lens with variable focal length (or variable optical power) for focusing, instead of moving the focus lens, the liquid crystal focus lens is adjusted electronically to provide 5-20 or more different focal lengths for the autofocus images. After capture, the 5-20 or more autofocus images are analyzed for contrast to determine the focus lens condition that delivers the image with the highest contrast which is deemed the best focus condition. In the analysis, focus values are generated for each autofocus image based on the level of contrast present. The focus lens is then returned to the focus condition that produced the autofocus image with the highest contrast, or an interpolated position between two or more of the autofocus images, before a final image is captured and stored. This method of autofocusing is known as the “hill climb method” because it generates a sequence of focus values that increase in level until they pass over a peak, i.e., a “hill”.
“Through-the-lens” autofocus systems can be very accurate since they measure focus quality directly from autofocus images captured with the same high quality taking lens that is used to capture the final image. However “through the lens” autofocus systems can also be very slow due to the many movements of the focusing lens required and the many autofocus images that must be captured and analyzed. This slowness in time-to-focus contributes to the objectionable delay perceived by the user between the time when the capture button is pressed and the image is actually captured which is known as shutter lag. It is desired to reduce shutter lag.
To reduce the time-to-focus, autofocus images are typically sub-sampled wherein only a portion of the available pixels are captured in the autofocus images e.g. as in a region of interest in the image such as where a face has been detected, to reduce the number of pixels that have to be analyzed. Typically, autofocus images are comprised of all the different types of pixels present including red, green and blue such as are present on an image sensor that has a Bayer pattern of colored pixels. For the case of an image sensor that has some pixels that are more sensitive to light such as panchromatic pixels that absorb a wider portion of the visible spectrum, autofocus images can be comprised of just the more sensitive pixels to enable shorter exposures and faster capture of the autofocus images. In addition, the pixels that are used to capture the autofocus images may be binned to increase the effective size of the pixels to increase sensitivity and enable shorter exposure times. Finally, the focusing lens must move very rapidly fast between focus positions or focal lengths. For an autofocus system with a liquid crystal focus lens, the time to focus is substantially limited by the relatively slow response (response times of 0.2 to 2.0 sec for a 10 diopter change) of the liquid crystal focus lens as it changes focal lengths over the range of different focus positions evaluated during the autofocus process.
A flow diagram of a conventional “hill climbing” contrast autofocus process is shown in FIG. 1 for gathering the autofocus images. FIG. 2, is an illustration of the relationship between the focus evaluation values obtained from analyzing the autofocus images and the lens position. In FIG. 2, the abscissa indicates the focusing position of a moveable focus lens (which is related to the focal length or optical power of the lens assembly) along the optical axis of the lens or the focal length of a fixed liquid crystal focus lens, the ordinate indicates the focusing evaluation value relative to a particular focus position P.
In FIG. 1, the autofocus process is begun when the user presses the capture button although other methods can be employed to initiate the autofocus process such as detecting movement of the camera. In a “whole way” autofocus evaluation, the complete set of autofocus images are captured and stored before the autofocus images are evaluated or analyzed for contrast. Wherein the number of autofocus images captured is determined by the depth of field of the lens, so that a lens with a greater depth of field requires fewer autofocus images than a lens with a lesser depth of field.
FIG. 3A shows a typical PRIOR ART liquid crystal lens 300. The liquid crystal lens 300 is comprised of a single thick liquid crystal lens 330 of thickness d with electrodes 310 and 320 on either side. Wherein the electrodes are comprised of layers of transparent electrically conductive material that has been coated onto a transparent substrates. The electrodes and the associated transparent substrates can be planar as shown or curved to modify the optical power of the liquid crystal lens. By applying a voltage V1 across the electrodes, the optical power of the lens can be caused to change. The electrodes can also be modified to produce different electric fields that act on the liquid crystal material to change the optical power over the surface of the liquid crystal lens. Various methods of producing lenses with controllable optical power have been disclosed in the patent literature. An example of a liquid crystal lens with controllable optical power can be found in U.S. Pat. No. 4,190,330. In addition, since liquid crystal materials are polarization sensitive, the liquid crystal material 330 can be comprised of 2 layers of liquid crystal materials with orthogonal molecular orientations separated by a glass substrate, that are operated in unison as disclosed in U.S. Pat. No. 4,572,616 so that the liquid crystal lens acts on all the polarization states of the light passing through the lens. A discussion of the evolution of liquid crystal lenses can be found in an article by C. W. Fowler and E. S. Pateras, “Liquid Crystal Lens Review” published in Ophthalmic and Physiological Optics, 1990, Vol. 10, pages 186-194.